In downlink transmission in wireless communications systems, such as an LTE-A system, there is reference signal for UE (user equipment) to perform channel estimation for demodulation of PDCCH (Physical downlink control channel) and other common channels as well as for measurement and some feedbacks, which is Common/Cell-specific Reference Signal (CRS) inherited from the Rel-8/9 specification of E-UTRA (Evolved Universal Terrestrial Radio Access). Dedicated/De-modulation reference signal (DMRS) can be transmitted together with the PDSCH (Physical downlink shared channel) channel in Rel-10 (Release-10) of E-UTRA. DMRS is used for channel estimation during PDSCH demodulation.
In Rel-10, CSI-RS (Channel Status Indication Reference Signal or Channel State Information Reference Signal) is introduced in addition to CRS (common reference signal) and DMRS (dedicated demodulation reference signal). CSI-RS is used for Rel-10 UEs to measure the channel status, especially for multiple antennas cases. PMI/CQI/RI and other feedbacks may be based on the measurement of CSI-RS for Rel-10 and beyond UE, wherein PMI is the precoding matrix indicator, CQI is channel quality indicator, and RI is rank indicator of the precoding matrix. CSI-RS in Rel-10 can support up to 8 transmission antennas while CRS can only support maximal 4 transmission antennas in Rel-8/9. The number of CSI-RS antenna ports can be 1, 2, 4, and 8. In addition, to support the same number of antenna ports, CSI-RS has much less overhead due to its low density in time and frequency.
Generally speaking, in OFDM (Orthogonal Frequency Division Multiplex) system, the frequency bandwidth is divided into multiple subcarriers in frequency domain. In time domain, one subframe is divided into multiple OFDM symbols. The OFDM symbol may have cyclic prefix to avoid the inter-symbol interference due to multiple path delay. One resource element is defined by the time-frequency resource within one subcarrier and one OFDM symbol. Reference signal and other signal such as data channel PDSCH, control channel PDCCH are orthogonal and multiplexed in different resource elements in time-frequency domain.
In OFDM system, signals are modulated and mapped into resource elements, after inverse Fourier transform per each OFDM symbol, the signals in frequency domain is transformed into the signals in time domain, and is transmitted with added cyclic prefix to avoid the inter-symbol interference.
A CSI-RS pattern is signaled to UE by RRC (radio resource control) signaling to support up to 8 transmission antennas. The CSI-RS pattern is cell-specific and hence common for all the Rel-10 UE, and it is repeated periodically with a duty cycle and a subframe offset.
To reduce the interference from and to CSI-RS of neighbor cells, muting of certain resource elements is performed in the PDSCH transmission. In other words, the PDSCH transmission transmits nothing (e.g., a zero power transmissions) in the muted resource elements indicated by a muting pattern. There is signaling to inform Rel-10 UE of the muting pattern. Hence Rel-10 UE discards the muted resource elements in the reception of PDSCH.